Its an old problem.  You need to consider what you want the data for, and the accuracy that you require. Actually crustaceans don't differ all that much between species. See paper by Sangun et al. 2009. (link below).  You could use the lower and upper estimates for "b" and give a range of values for the abundance biomass.  That would probably be a better estimate anyway than obtained from a "point in time" estimate from specific data - because its going to vary from season to season.

http://www.medwelljournals.com/fulltext/?doi=javaa.2009.75.79

Sewa - as you know, crabs grow in size only upon molting, whereas they gain weight during inter-molts. Hence, size increasing stepwise but weight increasing (potentially) continuously, the correlation between size (carapace width) and weight cannot be "perfect".

Nonetheless, within the same species the correlation might be close enough to get at least a rough estimate of biomass, if you have the carapace width...

I am not quite sure, however, why you cannot simply dry the crabs and weigh them?

Good luck!

Martin

 Thanks Martin! I did not dry and weigh them unfortunately  and those crabs are in indonesia now. I just have carapace width and length  for them and mostly not in abundant number to calculate their own regression equation. So I am thinking to use regression equation from segara anakan lagoon but dont have equations for all species from there as well. Its complicated now.

Within 1 km it may be considered. Beyond that is tough to consider. That is why we should look into ecological studies. 

yeah - this looks like not an easy task...

I know that some colleagues use the size: weight ratio beyond species borders - I am not sure I like it. You could go for a test with species of which you have specimens at hand? ->size them, weigh them, and check whether the ratio (or the regression line) is the same for different species. If so, you might consider doing this for your species, too. If not, ...

Martin

 Yes i have few of them  so i shall try to do as Martin suggested. Thanks its eye opening. 

Its an old problem.  You need to consider what you want the data for, and the accuracy that you require. Actually crustaceans don't differ all that much between species. See paper by Sangun et al. 2009. (link below).  You could use the lower and upper estimates for "b" and give a range of values for the abundance biomass.  That would probably be a better estimate anyway than obtained from a "point in time" estimate from specific data - because its going to vary from season to season.

http://www.medwelljournals.com/fulltext/?doi=javaa.2009.75.79

Dear Sewa,

Even a quick answer to your question will depend on your context and on what assumptions are you willing to make.

Assumptions: You could estimate the weight of your crabs by using a regression based on the width and weight of a different set of crabs IF you assume (and prove) that all crabs (including yours) belong to the same population or at least to a population with the same statistical parameters (and within a reasonable time-span), and also that your crabs are or can be considered a random sample of the other (larger?) population you are involving in your study. The larger the number of replicates the better for your estimation. If you cannot do that, you may only be able to explain your problems and still do a regression arguing that it is the best you can do.

Context: If you are conducting a PhD research, the important thing for your committee might be that you are in fact aware of the entire problem and that you are able to find the best (professional, scientific) solution possible. However, if you are thinking on writing an article for a high profile journal (Q1 or Q2)….then, you are in trouble.

Abrazos

Ale

There is a decent amount of literature on this with respect to morphometric changes (allometric growth). A good place to start is in the lobster literature, http://www.nrcresearchpress.com/doi/abs/10.1139/Z10-008#.Vx9nbmjF_zM. Do a google scholar search on morphometry lobster growth. I know similar comparisons have been made for several commercial crab species as well.